Substantial future oil well drilling will be at depths between 15,000 and 30,000 feet where temperatures encountered can be 220.degree. C. Drilling fluids must be tailored to accommodate the nature of the formation stratum being encountered at the time. When the drilling reaches the producing formations, special concern is exercised. Preferentially, low solids content fluids are used to minimize possible productivity loss by solids plugging, enhance drilling rate and reduce sedimentation in inclined wells. Proper fluid density for balancing formation pressure may be obtained by using high salt concentration aqueous brines, while viscosity and filter loss control may be obtained by polymer addition.
The high temperatures, coupled with the desire for low solids content and preferably minimum added solids, require brine tolerant and high temperature stable polymers for viscosity and filtration control.
Current high density, clear brine systems utilize hydroxyethyl cellulose polymers and related materials as viscosifiers, but these are normally unstable at about 135.degree. C., and tend to cross-link and gel with time and temperature, thus causing various drilling operational problems.
In drilling for oil and gas, such as the Mobile Bay and Arun fields, the use of clear brine completion, workover, and packer fluids containing divalent calcium and zinc, can lead to precipitation of carbonates, plugging of wells, and subsequent loss of hydrocarbon productivity. Sodium bromide brines provide a possible alternative. Until the present, however, the use of clear brine fluids has not been fully developed because of the unavailability of viscosifying agents and additives to provide suitable rheological properties for removal of drilled solids and to minimize filtration losses. In addition, it is generally difficult to predict what effect the brine, particularly high concentrations thereof, will have on the thermal properties of the polymer.
Viscosifier/brine systems, which are stable in a 150.degree.-220.degree. C. range, and are suitable for high temperature, deep-drilling operations for oil and gas, oil recovery and other possible applications are known to the art. For example, U.S. patent application Ser. Nos. 490,901 and 490,623 disclose the addition of N-methylpyrrolidone in order to enhance the thermal stability of a vinyl sulfonate amide copolymer in a dense sodium bromide brine at 220.degree. C. However, a relatively high dosage of the added N-methylpyrrolidone is required. Additionally, U.S. Pat. No. 4,309,523 to Engelhardt et al, the entire disclosure of which is expressly incorporated herein by reference, describes drilling muds, i.e., drilling fluids which contain clay, water-soluble copolymers of acrylamidomethylpropanesulfonic acid salts and other components, such as salts, but does not refer to clear brine.
Thus, although some form of thermal stability of brine fluids has been found in the 150.degree.-220.degree. C. range, there is room for improvement. It is therefore an object of the present invention to provide an improved aqueous brine fluid particularly for use as a completion, workover, packer and drilling fluid.
Further, it is an object of this invention to overcome the deficiencies of the prior art.
It is another object of this invention to increase the thermal stability of polymeric viscosifiers in brine drilling fluids at high temperatures, e.g., in the range of 220.degree. C.
It is still another object of this invention to provide an aqueous brine fluid with enhanced thickening characteristics.
It is a further object of this invention to provide an aqueous brine fluid which is economically more desirable.
Additionally, it is an object of this invention to provide new and superior additive systems for brine fluids containing vinyl sulfonate amide copolymers.
Still further, it is an object of this invention to produce a new and improved aqueous brine fluid by the use of additives which increase the thermal stability of the brine fluid without having to chemically alter the brine fluid system.
These and other objects will become apparent from the specification which follows.